Download 2002

Applied Vegetation Science 5: 195-202, 2002
© IAVS; Opulus Press Uppsala.
- ARE INVADERS DISTURBANCE-LIMITED? CONSERVATION OF MOUNTAIN GRASSLANDS -
195
Are invaders disturbance-limited?
Conservation of mountain grasslands in Central Argentina
Petryna, L.1; Moora, M.2; Nuñes C.O.1; Cantero, J.J.1 & Zobel, M.2*
1Universidad
Nacional de Rio Cuarto, Facultad de Agronomia y Veterinaria, Estafeta Postal No. 9, 5800 Rio Cuarto,
Cordoba, Argentina; 2Institute of Botany and Ecology, Tartu University, Lai 40, Tartu 51005, Estonia;
*Corresponding author; Fax +3727376222; E-mail [email protected]
Abstract. Extensive areas in the mountain grasslands of central Argentina are heavily invaded by alien species from
Europe. A decrease in biodiversity and a loss of palatable
species is also observed. The invasibility of the tall-grass
mountain grassland community was investigated in an experiment of factorial design. Six alien species which are widely
distributed in the region were sown in plots where soil disturbance, above-ground biomass removal by cutting and burning
were used as treatments. Alien species did not establish in
undisturbed plots. All three types of disturbances increased
the number and cover of alien species; the effects of soil
disturbance and biomass removal was cumulative. Cirsium
vulgare and Oenothera erythrosepala were the most efficient
alien colonizers. In conditions where disturbances did not
continue the cover of aliens started to decrease in the second
year, by the end of the third season, only a few adults were
established. Consequently, disturbances are needed to maintain alien populations in tall-grass mountain grasslands. Burning also increased the species richness of native species. We
conclude that an efficient way to control the distribution of
alien species is to decrease grazing pressure while burning as
a traditional management tool may be continued.
Keywords: Alien species; Burning; Grazing; Invasibility;
Management; Species richness.
Nomenclature: Cantero & Bianco (1986).
Introduction
Invasions in grasslands are common and, in many
cases, have been associated with changes in grazing
regime (Sala et al. 1986; D’Antonio & Vitousek 1992;
Mack 1996). Argentinian grasslands that have evolved
under low grazing pressure have even shown an increase
in species diversity as a result of the introduction of
livestock (Sala et al. 1986), while cessation of grazing
may result in a decrease in diversity (Pucheta et al. 1998).
Rusch & Oesterheld (1997) found that in grasslands of
central-eastern Argentina, grazing increased species richness through the addition of exotic forbs, without reducing the richness and cover of the native flora. However,
one may currently observe the rapid extension of the
distribution area of several exotic species in the mountain
grasslands of central and southern Argentina, especially
where land use is, or has been, intensive (Mack 1996).
There is evidently a need to control the distribution and
abundance of aliens. For these reasons, one has to know
the mechanisms enhancing establishment and persistence
of alien species populations.
Following arrival in a certain locality, successful
establishment and subsequent expansion of the alien
species depends on local abiotic conditions and on biotic interactions such as competition from established
native plants, predation, parasitism and on the presence
of mutualistic organisms (Crawley 1987; Vitousek 1990;
Williamson 1999). Rejmánek (1996) showed that communities in mesic environments are more invasible as
xeric environments are not favourable for seedling establishment and wet habitats are characterized by fast
growth and high competitiveness of resident species.
Several pieces of evidence show that the number of
alien species is greater in more disturbed habitats
(Kitayama & Mueller-Dombois 1995; Burke & Grime
1996; Crawley et al. 1996; Rejmánek 1995, 1999;
Rejmánek & Richardson 1996; Morgan 1998), while
traditional management of semi-natural grasslands may
196
PETRYNA, L. ET AL.
inhibit invasions (Brabec & Pyšek 2000). Consequently,
the real pattern of successful invasions by exotic species
is dependent on local land use, which creates disturbance.
The vascular plant flora of the Cordoba Mountain
grasslands of central Argentina consists of 388 species,
46 of which are endemic and 25 are European invaders,
12 of which are common in mountain grasslands (Cantero
et al. 1999). The distribution of alien species is patchy –
there are localities where their abundance is remarkably
high and they are absent from other areas. Since natural
conditions are similar over the whole area, such a patchy
distribution of exotic species is either dependent on
dispersal limitation or land use. These grasslands are
distributed in relatively humid conditions so one may
assume that both grazing and fire are significant forces
shaping grassland communities (Milchunas et al. 1988;
Milchunas & Lauenroth 1993). Grazing pressure varies
over the area and may be relatively high in some places.
Burning is carried out spasmodically under relatively
little control and may, in some years, affect extensive
areas. In a few cases, grasslands on high plateaus have
been ploughed (Díaz et al. 1990).
The pattern of species richness in undisturbed mountain grassland vegetation showed that species richness
and composition of short-grass communities depended
on the species composition in the close neighbourhood,
while in the case of tall-grass communities such a dependence did not exist (Cantero et al. 1999). It was
concluded that in short-grass communities, local dispersal was an important process influencing the pattern of
community composition, since most of the seed rain
arrives only in the closest neighbourhood of mother
plants (Harper 1977; Legg et al. 1992; Poschlod et al.
1996; Brunet et al. 2000) and similarity between neighbouring stands indicates the exchange of diaspores. In
tall-grass communities, the similarity between neighbouring stands was not higher than mean similarity over
the whole study area. This observation was explained by
the higher intensity of local biotic interactions, such as
light competition, in communities with a tall canopy
(standing crop ca. 600 g.m–2), which may inhibit the
establishment of diaspores arriving from the close neighbourhood. At the same time, disturbances like grazing
and burning may open the canopy and make the communities more invasible. We hypothesized that the establishment of European aliens in tall-grass communities is
primarily microsite limited and local disturbances, opening the canopy, will inevitably result in successful invasion.
This hypothesis was tested in an experiment of a
factorial design, where seeds of the six most abundant
alien species in the region were sown in the plots.
Removal of the above-ground biomass by cutting, soil
disturbance (aimed to mimic herbivory and trampling,
respectively), and burning were used as treatments. In
particular, we were interested in the following questions:
1. Is the establishment of aliens in mountain grasslands microsite limited?
2. Do different kinds of disturbances have differential effects on the establishment of aliens?
3. How do the disturbances and the establishment of
aliens influence the number of native species?
4. What is the optimal way of grassland management, resulting in the lowest cover of alien species?
Methods
Study area
The central Argentinan mountains cover an area of
ca. 35 000 km2 between 29∞ S and 33∞ 30' S, rising from
500 to 3000 m a.s.l. Parent rock is an ancient crystalline
(gneiss) peneplain intruded by granite batholiths; its
early development can be traced to the pre-Cambrian
and early Paleozoic periods (Gordillo & Lencinas 1970).
The remains of an eroded cretaceous sedimentary cover
of conglomerates and sandstone can be found locally.
Major plant formations are distributed along different
altitudinal belts, with forest at lower levels and perennial tussock grasses at higher altitudes. The climate in
the region is humid, with short, cool summers and long,
cold winters. The mean annual temperature is 10 ∞C at
1500 m and annual rainfall ranges between 980 and
1400 mm.
The study was carried out in the Cordoba Mountains, 32∞42' S, 32∞53' S and 64∞49' W, 65∞ W, in a
grassland belt of nearly 8 ¥ 104 ha. The study area is
relatively geographically and climatically homogeneous. The major geomorphic surfaces are plains, slopes
and valleys.
The vegetation is a mosaic of three vegetation types:
tall tussock grasslands with predominating Festuca
hieronymi and/or Deyeuxia hieronymi (tall-grass) mostly
on mountain plains with relatively deep soil (20 - 30 cm),
dry grasslands on slopes with predominating Sorghastrum pellitum (short-grass) where the soil layer is thin
(10 - 20 cm) and wet turf grassland with predominating
Poa stuckertii in valleys (Cantero et al. 1996). In the
study area, these vegetation types occupy 17, 77 and 6%
respectively.
In the last 100 years, the grasslands have been grazed
by domestic animals with moderate grazing pressure
(e.g. 0.2 - 0.3 cattle.ha–1). The field observations showed
(Cantero, pers. obs.) that the current grazing pressure
varies between 0.2 and 1.0 animal.ha–1 and that cattle
spend ca. 36% of their feeding time in tall-grass, 47 % in
short-grass and 17 % in wet turf communities. In
- ARE INVADERS DISTURBANCE-LIMITED? CONSERVATION OF MOUNTAIN GRASSLANDS historic and prehistoric times, grasslands evolved under
grazing by native herbivores (guanaco = Lama guanicoe).
However, as Mack (1996) pointed out, guanaco were
too light to affect selection in grasses through trampling.
Due to both inability to produce axillary buds and well
exposed erect tillers caespitose grasses, predominating
in the tall-grass community, are often more vulnerable
to grazing than rhizomatous ones. The soil seed bank of
a tall-grass community – in the same area where the
field experiment was conducted – was described by
Amuchastequi et al. (1998). Seed density (467 seeds.m–
2) and number of species (23 species.m–2) in the seed
bank were low. Most of the species in the bank were
perennials (17) and no alien species were represented.
Field experiment
In June 1997, seeds of the six most frequent alien
species (Table 1) were collected from the Cordoba
mountain grasslands and weighed – the mean weight is
calculated on the measurement of 100 seeds. The height
of ten specimens of each species was also measured. In
September 1997, 100 plots (1 m ¥ 1 m) were located
randomly within a homogeneous 0.5-ha patch of the
tall-grass grassland. In mid-October 1997, an experiment of factorial design was established using three
types of disturbance as treatments: burning of the aboveground biomass, removing of the standing crop with
scissors at a height of 10 cm, and loosening of the soil
surface to 10 cm depth using a garden fork. All combinations of these treatments (nine) were randomly assigned to different plots. There were ten replicates per
treatment, thus the experiment included 90 plots. On
October 28, 1997 the plots were sown with a mixture of
Table 1. Alien species sown in experimental plots. Form = Life
form: B = biennial; P = perennial; G = grass; L = legume; F =
non-legume forb. Weight = mean seed weight (mg); Height =
mean height when growing in a tall-grass stand (cm); Disp. =
seed dispersal type: A = anemochorous; E = epizoochorous; U =
unspecified. The six species in the table were the most abundant
from a pool of 25 Euroasiatic alien species, the others being:
Centunculus minimus, Ciclospermum leptophyllum, Duchesnea indica,
Eleusine indica, Elytrigia repens, Euphorbia peplus, Geranium dissectum,
Hirchsfeldia incana, Lithrum hyssopyfolia, Medicago lupulina, Mentha
pulegium, Poa annua, Polygonum convolvulus, Rorippa nasturtiumaquaticum, Rumex crispus, Sonchus oleraceus, Taraxacum officinale,
Tragopogon dubius, Viola metajaponica.
Form Weight Height Disp.
Cirsium vulgare (Asteraceae)
Lolium multiflorum (Poaceae)
Lotus corniculatus (Fabaceae)
Oenothera erythrosepala (Onagraceae)
Trifolium repens (Fabaceae)
Verbascum thapsus (Scrophulariaceae)
BF
BG
PL
BF
PL
BF
1.65
2.47
1.60
0.28
0.72
0.15
100
25
25
80
30
70
A
E
U
E
U
E
197
seeds applied to the central 0.5 m ¥ 0.5 m area of each
plot. We followed Burke & Grime (1996), leaving the
rest of the plot area as a ‘buffer’ zone where the vegetation received the same disturbance treatment as the
central subplot immediately adjacent to it, thus reducing
the possibility of edge effects. Apart from some C.
vulgare individuals none of the sown species were present
in the experimental patch. Each 0.5 m ¥ 0.5 m subplot
received 0.15 g of seeds of each of the six species, i.e.
fewer seeds of the species with heavier seeds. The
resulting difference in sowing rates between large and
small seeded species reflects a widespread inverse relationship between the numbers of seeds produced and
seed size and between numbers of seed produced and
rates of establishment (Burke & Grime 1996). All 90
plots received the same seed mixture. Seeds were manually sprinkled over the central subplot. To minimize
accidental additions, a 0.5 m ¥ 0.5 m ¥ 0.90 m bottomless box was used for spreading the seeds. Ten additional plots were untreated and unsown, these plots were
later compared to untreated and sown plots. The floristic
composition of the 0.5 m ¥ 0.5 m subplots and the 1 m ¥
1 m plots was recorded in February 1998, March 1999
and September 1999. Species presence was defined as
the occurrence of living shoots rooted in the plot. The
cover percentage of all vascular plant species in plots
was estimated visually according to the following scale:
10 = 91 - 100 %; 9 = 81 -90 %;
= 51 - 60 %;
5 = 41 - 50 %;
= 11 - 20 %;
1 = 1 - 10%.
8 = 71 - 80 %;
4 = 31 - 40 %;
7 = 61 - 70 %; 6
3 = 21 - 30 %; 2
Statistical analysis
The effects of biomass removal, soil disturbance,
and burning (all with two levels) on the number of plant
species (alien and native species) and on the cover of
alien species (total cover and cover of particular species) in plots was analysed by four-way repeated measures analysis of variance (ANOVAR). The effect of
sowing on the non-disturbed plant community was studied by two-way ANOVAR. In both analyses, time was
considered as the repeated measure factor. If necessary
the data was approximated to normality of residuals by
log-transformation [log (x + 1)]. Comparison between
means was made with the Tukey-Kramer test. For all
analyses, the Windows version of STATISTICA (Anon.
1999) was used.
198
PETRYNA, L. ET AL.
Results
Intact communities
Sowing of six alien species in the undisturbed tallgrass community did not increase the number of alien
species in time, compared to the unsown treatment (P >
0.05). Only two or three individuals of O. erythrosephala,
L. corniculatus and T. repens established in the plots.
Effect of disturbances on sown plots
Soil disturbance, biomass removal and burning significantly increased the number of alien species (Fig. 1,
Table 2). After sowing, the number of alien species
decreased in time. Burning and above-ground biomass
removal alone both significantly increased the number
of alien species. When both treatments were applied
together, no additive effect was evident. The alien species
number decreased significantly between the first and
second and the second and third seasons when the
above-ground biomass was removed. For all other treatment combinations, a significant decline of alien species
number in time was evident only between the first and
second seasons.
The total cover of aliens was significantly positively
dependent on soil disturbance, biomass removal and
burning (Table 3). Both biomass removal and burning
alone increased the alien species cover to a similar
extent but when applied together, no additive effect was
observed. In disturbed soil plots, the decrease of the
cover of aliens was more rapid than in non-disturbed
plots, but the cover of aliens still remained higher in soil
disturbed than in non-disturbed plots at the end of the
Table 2. ANOVA results. The effect of soil disturbance (Sd),
above-ground biomass removal (Br) and burning (Fi) as fixed
factors and time (Ti) as repeated measures factor on two
dependent variables: alien species number in plot (after first,
second and third seasons) and native species number in plot
(initial and after first, second and third seasons).
Source of var.
Alien species number
df
F
P
Native species number
df
F
P
Sd
Br
Fi
Ti
Sd*Br
Sd*Fi
Br*Fi
Sd*Ti
Br*Ti
Fi*Ti
Sd*Br*Fi
Sd*Br*Ti
Sd*Fi*Ti
Br*Fi*Ti
Sd*Br*Fi*Ti
1
1
1
2
1
1
1
2
2
2
1
2
2
2
2
1
1
1
3
1
1
1
3
3
3
1
3
3
3
3
58.03
56.38
54.76
82.63
0.30
2.32
47.0
1.12
1.10
2.54
0.58
2.93
2.23
3.44
1.76
.000
.000
.000
.000
.588
.132
.000
.329
.355
.082
.449
.057
.111
.035
.176
0.03
0.03
1.3
62.13
0.21
2.49
0.87
0.31
1.06
5.07
1.98
0.21
0.143
2.77
1.16
.864
.864
.258
.000
.650
.119
.355
.819
.369
.002
.164
.888
.934
.043
.325
Table 3. ANOVA results. The effects of soil disturbance (Sd),
above-ground biomass removal (Br) and burning (Fi) as fixed
factors and time (Ti) as repeated measures factor on three
dependent variables: total cover of alien species, cover of
Cirsium vulgare and cover of Oenothera erythrosepala.
Source
of var.
df
Total cover
of aliens
F
P
Sd
Br
Fi
Ti
Sd*Br
Sd*Fi
Br*Fi
Sd*Ti
Br*Ti
Fi*Ti
Sd*Br*Fi
Sd*Br*Ti
Sd*Fi*Ti
Br*Fi*Ti
Sd*Br*Fi*Ti
1
67.1
1 40.81
1 54.24
1 161.12
1
3.82
1
0.68
1 18.35
1 12.97
1
2.63
1 13.62
1
0.56
1
0.01
1
2.63
1
3.24
1
1.82
0.00
0.00
0.00
0.00
0.05
0.41
0.00
0.00
0.11
0.00
0.45
0.93
0.11
0.08
0.18
Cover
of Cirsium
F
P
17.33
22.04
4.15
24.11
3.46
0.01
4.15
4.90
4.09
0.45
0.64
0.75
0.08
1.12
0.01
0.00
0.00
0.04
0.00
0.07
0.93
0.04
0.03
0.05
0.50
0.45
0.39
0.77
0.29
0.92
Cover
of Oenothera
F
P
13.92
12.03
35.47
8.96
2.27
1.89
4.12
0.09
0.86
11.60
1.55
0.27
2.40
0.10
0.01
0.00
0.00
0.00
0.00
0.14
0.17
0.05
0.76
0.36
0.00
0.22
0.61
0.13
0.76
0.92
third season. The decline in the cover of aliens in time
was more rapid in burned than in unburned plots. Again,
the cover of aliens still remained higher in burned plots
at the end of the third season.
Data on only five alien species was considered in per
species analysis, since V. thapsus establishment was
very low in experimental plots. C. vulgare and O.
erythrosepala established more successfully so 2-yr
data was used for ANOVAR (Table 3). For the rest of
the species ANOVA was applied for the 1st year data
(Table 4). The cover of C. vulgare, O. erythrosepala, L.
corniculatus and T. repens increased significantly after
soil disturbance, biomass removal and burning. In the
case of C. vulgare and O. erythrosepala a decrease of
cover in time occurred in the second and third years.
Cover of L. perenne significantly increased after soil
disturbance and burning, while biomass removal had no
effect. There was a significant interaction between bioTable 4. ANOVA results. The effects of soil disturbance (Sd),
above-ground biomass removal (Br) and burning (Fi) as fixed
factors on three dependent variables: cover of Lolium
multiflorum, of Lotus corniculatus and of Trifolium repens
after the first year.
Source
of var.
df
Sd
Br
Fi
Sd*Br
Sd*Fi
Br*Fi
Sd*Br*Fi
1
1
1
1
1
1
1
Lolium
F
P
16.2
0.04
8.27
0.92
0.04
0.33
0.33
0.00
0.85
0.01
0.34
0.85
0.57
0.57
Cover
Lotus
F
P
12.71
5.65
5.65
3.18
3.18
28.59
1.41
0.00
0.02
0.02
0.08
0.08
0.00
0.24
Trifolium
F
P
9.7
7.43
8.53
0.15
1.86
3.07
0.34
0.00
0.01
0.00
0.70
0.18
0.08
0.56
- ARE INVADERS DISTURBANCE-LIMITED? CONSERVATION OF MOUNTAIN GRASSLANDS -
199
Fig. 1. The effects of soil disturbance (A), burning (B) and above-ground biomass removal (C) on the alien species number during
the experiment.
mass removal and burning in the case of C. vulgare and
L. corniculatus – both treatments increased the cover of
these species but no additive effect was observed.
In the case of C. vulgare, interactions between soil
disturbance and time, and between biomass removal
and time were significant. C. vulgare had higher cover
in the first year in those experimental plots where soil
disturbance or biomass removal were applied. In the
second year, however, these differences vanished. In the
case of O. erythrosepala, there was a significant interaction between burning and time – the cover of O.
erythrosepala started to decrease more rapidly in previously burned plots.
Native species richness and disturbances
Soil disturbance, biomass removal and burning had
no significant main effects on native species richness.
There were significant interactions, however, between
burning and time (F = 5.07, P < 0.01) and between
biomass removal, burning and time (F = 2.77, P = 0.04).
Burning resulted in an increase in native species richness during the first two years after experimental treatment, while this effect tended to vanish when biomass
removal was applied before burning (Fig. 2).
200
PETRYNA, L. ET AL.
Fig. 2. The effects of soil disturbance (A), burning (B) and
above-ground biomass removal (C) on the native species
number during the experiment.
Discussion
Major transformations of Argentinian vegetation are
continuing (Mack 1996). There is an obvious need to
provide farmers with a method of grassland management for conservation, resulting in the lowest proportion of European alien species. The results of the field
experiment confirmed that the establishment of aliens in
central Argentinian mountain grasslands depends on
local disturbances. Sowing alone resulted in the establishment of only a few individuals, while all three kinds
of disturbances – burning, above-ground biomass re-
moval and soil disturbance – considerably increased the
success of aliens. As the native tall-grass community is
characterized by a remarkably high standing crop, the
positive effect of disturbances on the establishment of
invaders is in accordance with both theory (Huston
1994; 1999) and case studies (Foster & Gross 1998;
Stampfli & Zeiter 1999; van der Putten et al. 2000;
Foster 2001). The positive effects of the three types of
disturbances on the establishment of aliens were similar. The effects of biomass removal and soil disturbance
were cumulative – the highest number of alien species,
as well as the highest total cover of aliens, were observed in plots where both treatments were applied.
Biomass removal alone reduced the level of light competition, but did not produce suitable gaps for regeneration from seeds, and additional soil disturbance enhanced seedling establishment. As above-ground biomass removal and soil disturbance, when applied together, mimic grazing, one may expect that overgrazing
is the most effective means of enhancing establishment
of alien species. The present observations (Cantero unpubl.) show that the ‘limit of disturbance’ is a grazing
pressure of ca. 0.5 animal.ha–1.yr–1.
Among the sown species, C. vulgare was certainly
the most efficient invader. It is a species with a good
capability for wind dispersal and with a persistent seed
bank (Grime et al. 1988; Doucet & Cavers 1996;
Thompson et al. 1997). The height of C. vulgare –
flowering stems were up to 2 m high – also potentially
makes it a good competitor for light. As it is not reproducing clonally, regeneration from seeds is still the
critical life stage and, in a closed turf, the creation of
gaps by local disturbances is needed. The second successful species, O. erythrosepala, is also a good competitor for light, since adults are often more than 1 m tall
and seedlings are shade tolerant.
After the experimental disturbances at the beginning
of the experiment, the plots were further protected from
grazing and burning. In the experimental plots, there
was a clear decline in the number of species and cover of
aliens in time, and relatively few adult individuals were
established at the end of the third year. Considering the
remarkably low establishment rate of aliens in nondisturbed communities, one may conclude that the regeneration or even the performance, of aliens is strongly
dependent on disturbances. Consequently, there is a
good chance of restoring communities which have been
invaded by aliens, but still have native species, by
reducing the level of disturbances.
As predicted by the dynamic equilibrium theory
(Huston 1994, 1999), disturbances increased the richness of native species in the productive tall-grass community, though this effect was most evident in the
burned plots. Since Deyeuxia hieronymi and Festuca
- ARE INVADERS DISTURBANCE-LIMITED? CONSERVATION OF MOUNTAIN GRASSLANDS hieronymi predominate there, many native species are
probably suppressed by competition and their regeneration is enhanced by disturbances, which open the canopy.
Fire is evidently the most efficient means of suppressing
dominants for a time required for the establishment of
various native species. In certain circumstances, fire
may even be the factor limiting the occurrence of alien
species (Smith & Knapp 1999), though the effect may
vanish in cases of high propagule input (Smith & Knapp
2001).
The optimal conservation-oriented management of
the mountain grasslands of central Argentina requires,
first of all, regulation of grazing pressure. Lowering of
the grass canopy and creation of gaps by cattle has a
cumulative positive effect on the establishment of aliens. The observed limit of the grazing pressure, which
starts to create visible disturbances in the vegetation,
was 0.5 animal.ha–1.yr–1, which is less than normal in
the southern part of the Cordoba mountains. As there are
areas heavily invaded by aliens, one may assume that
the cattle are distributed unevenly over the area. Consequently, fencing is needed to make the grazing pressure
more uniform. At present, the recovery of the natural
grassland community after the reduction of grazing
pressure is relatively easy and fast, unless slope erosion
has taken place. Burning can be continued because,
although it temporarily increases the number of aliens, it
also enhances the regeneration of the native subordinate
species.
Acknowledgements. The University of Rio Quarto and Tartu
University (TBGBO 0553) financed this study. We would like
to thank I. Part, P. Pyšek and an anonymous referee for
comments on the first draft of the manuscript.
References
Amuchastegui, A., Cantero, J.J., Nuñez, C. & Petryna, L.
1998. Influence of different grazing intensities on the seed
bank of natural grasslands at Central Argentina mountains. Argent. Bot. Congr. 26: 224.
Brabec, J. & Pyšek, P. 2000. Establishment and survival of
three invasive taxa of the genus Reynoutria (Polygonaceae)
in mesic mown meadows: a field experimental study.
Folia Geobot. 35: 27-42.
Brunet, J., von Oheimb, G. & Diekmann, M. 2000. Factors
influencing vegetation gradients across ancient-recent
woodland borderlines in southern Sweden. J. Veg. Sci. 11:
515-524.
Burke, M.J.W. & Grime, J.P. 1996. An experimental study of
plant community invasibility. Ecology 77: 776-790.
Cantero, J.J. & Bianco, C.A. 1986. Las plantas vasculares del
suroeste de la Provincia de Cordoba. III. Catalogo
201
preliminar de las especies. Rev. Univ. Nac. Rio Cuarto 6:
5-52.
Cantero, J.J., Cantero, A. & Cisneros, J.M. 1996. La vegetación
de los paisajes hidrohalomorficos del centro de Argentina.
Universidad Nacional de Rio Cuarto, Rio Cuarto, AR.
Cantero, J.J., Pärtel, M. & Zobel, M. 1999. Is species richness
dependent on the neighbouring stands? An analysis of the
community patterns in mountain grasslands of central
Argentina. Oikos 87: 346-354.
Crawley, M.J. 1987. What makes a community invasible? In:
Crawley, M.J., Edwards P.J. & Gray A.J. (eds.) Colonisation, succession and stability, pp. 429-454. Blackwell
Scientific Publication, Oxford, UK.
Crawley, M.J., Harvey, P.H. & Purvis, A. 1996. Comparative
ecology of the native and alien floras of British Isles. Phil.
Trans. R. Soc. Lond. B 351: 1251-1259.
D’Antonio, C.M. & Vitousek, P.M. 1992. Biological invasions by exotic grasses, the grass/fire cycle, and global
change. Annu. Rev. Ecol. Syst. 23: 63-87.
Díaz, S., Acosta, A. & Cabido, M. 1990. Spatial patterns and
diversity in a post-ploughing succession in high plateau
grasslands, Pampa de San Luis, Cordoba, Argentina. Stud.
Geobot. 10: 3-13.
Doucet, C. & Cavers, P.B. 1996. A persistent seed bank of the
bull thistle Cirsium vulgare. Can. J. Bot. 74: 1386-1391.
Foster, B.L. 2001. Constraints on colonization and species
richness along a grassland productivity gradient: the role
of propagule availability. Ecol. Lett. 4: 530-535.
Foster, B.L. & Gross, K.L. 1998. Species richness in a successional grassland: effects of nitrogen enrichment and plant
litter. Ecology 79: 2593-2602.
Gordillo, C. & Lencinas, A. 1979. Las sierras pampeanas de
Cordoba y San Luis. In: Simposio Geologia Argentina, pp.
577-650. Academia Nacional de Ciencias, Cordoba, AR.
Grime, J.P., Hodgson, J.G. & Hunt, R. 1988. Comparative
plant ecology. Unwin Hyman, London, UK.
Harper, J.L. 1977. Population biology of plants. Academic
Press, London, UK.
Huston, M.A. 1994. Biological diversity. Cambridge University Press, Cambridge, UK.
Huston, M.A. 1999. Local processes and regional patterns:
appropriate scales for understanding variation in the diversity of plants and animals. Oikos 86: 393-401.
Kitayama, K. & Mueller-Dombois, D. 1995. Biological invasions on an oceanic island mountain: Do alien plant species
have wider ecological ranges than native species? J. Veg.
Sci. 6: 667-674.
Legg, C.J., Maltby, E. & Proctor, M.C.F. 1992. The ecology of
severe moorland fire on the North York Moors: seed
distribution and seedling establishment of Calluna vulgaris. J. Ecol. 80: 737-752
Mack, R.N. 1996. Temperate grasslands vulnerable to plant
invasions: characteristics and consequences. In: Drake, J.A.,
Mooney, H.A., di Castri, F., Groves, R.H., Kruger, F.J.,
Rejmánek, M. & Williamson, M. (eds.) Biological invasions. A global perspective, pp. 155-179. Wiley, Chichester, UK.
Milchunas, D.G. & Lauenroth, W.K. 1993. Quantitative effects of grazing on vegetation and soils over a global range
202
PETRYNA, L. ET AL.
of environments. Ecol. Monogr. 63: 327-366.
Milchunas, D.G., Sala, O.E. & Lauenroth, W.K. 1988. A
generalized model of the effects of grazing by large herbivores on grassland community structure. Am. Nat. 132:
87-106.
Morgan, J.W. 1998. Patterns of invasion of an urban remnant
of a species-rich grassland in southeastern Australia by
non-native plant species. J. Veg. Sci. 9: 181-190.
Poschlod, P., Fischer, S. & Kiefer, S. 1996. A coenotical
approach of plant population viability analysis on successional and afforested calcareous grassland sites. In: Settele,
J., Margules, C., Poschlod, P. & Henle, K. (eds.) Species
survival in fragmented landscapes, pp. 219-229. Kluwer,
Dordrecht, NL.
Pucheta, E., Cabido, M., Diaz, S. & Funes, G. 1998. Floristic
composition, biomass, and aboveground net plant production in grazed and protected sites in a mountain grassland
of central Argentina. Acta Oecol. 19: 97-105.
Rejmánek, M. 1995. What makes a species invasive? In:
Pyšek, P., Prach, K., Rejmánek, M. & Wade, P.M. (eds.)
Plant invasions, pp. 3-13. SPB, The Hague, NL.
Rejmánek, M. 1996. Invasibility of plant communities. In:
Drake, J.A., Mooney, H.A., di Castri, F., Groves, R.H.,
Kruger, F.J., Rejmánek, M. & Williamson, M. (eds.) Biological invasions. A global perspective, pp. 369-388. Wiley,
Chichester, UK.
Rejmánek, M. 1999. Invasive plant species and invasible
ecosystems. In: Sandlund, O.T., Schei, P.J. & Viken, A.
(eds.) Invasive species and biodiversity management, pp.
79-102. Kluwer Academic Publishers, Dordrecht, NL.
Rejmánek, M. & Richardson, D.M. 1996. What attributes
make some plant species more invasive? Ecology 77:
1655-1661.
Rusch, G.M. & Oesterheld, M. 1997. Relationship between
productivity, and species and functional group diversity in
grazed and non-grazed Pampas grassland. Oikos 78: 519526.
Sala, O.E., Oesterheld, M., Leon, R.J.C. & Soriano, A. 1986.
Grazing effects upon plant community structure in
subhumid grasslands of Argentina. Vegetatio 67: 27-32.
Smith, D.M. & Knapp, A.K. 1999. Exotic plant species in a
C4-dominated grassland: invasibility, disturbance, and community structure. Oecologia 120: 605-612.
Smith, M.D. & Knapp, A.K. 2001. Size of the local species
pool determines invasibility of a C4-dominated grassland.
Oikos 92: 55-61.
Stampfli, A. & Zeiter, M. 1999. Plant species decline due to
abandonment of meadows cannot easily be reversed by
mowing. A case study from the southern Alps. J. Veg. Sci.
10: 151-164.
Thompson, K., Bakker, J.P. & Bekker, R.M. 1997. Soil seed
banks of North-West Europe: Methodology, density and
longevity. Cambridge University Press, Cambridge, UK.
van der Putten, W. et al. 2000. Biodiversity experiments on
abandoned arable land along climate transects. Oecologia
124: 91-99.
Vitousek, P.M. 1990. Biological invasions and ecosystem
processes: towards an integration of population biology
and ecosystem studies. Oikos 57: 7-13.
Williamson, M. 1999. Invasions. Ecography 22: 5-12.
Received 6 April 2001;
Revision received 27 December 2001;
Accepted 27 December 2001.
Coordinating Editor: J. Pfadenhauer.